"Resists" are materials that are resistant to the dissolving action of certain solvents. Photoresists are resists that are also light-sensitive in that exposure to some form of actinic radiation causes them to at least partially change chemically from one to the other of two alternative forms. One of these forms is resistant to certain solvents and the other is soluble in those solvents.
Most photoresists change from a soluble to an insoluble form when they are exposed to light. These are known as "negative" photoresists because the areas of photoresist remaining after development of an image are those that have been exposed to light. The parts that have been hardened are the negative of the shaded parts of the image or photomask.
Some photoresists are of the "positive" acting type. That is, when they are exposed to light, they change from being soluble in one type of solvent to being soluble in a different type of solvent and development of the photoresist occurs where light has impinged thereon.
Although some photoresists comprise resinous materials which are photosensitive without the addition of other substances, most photoresists include a "sensitizer" which increases their light-sensitivity to a level that is suited for practical use.
Most previously used photoresist compositions include one or more organic solvents as vehicles and, after exposure, they are also developed with organic solvents. This limits their use somewhat since they cannot be used on substrates which are attacked by the developer solvents. Also, organic solvents are both fire and health hazards. Special precautions and special equipment must be used in handling them.
The present invention provides a novel photoresist composition which is soluble in water. When a film of the composition is dried, the dried film is insoluble in water. However, the resist is a positive-working type and after the film is exposed to ultra-violet light, the portions exposed to light become soluble in warm water.